The present disclosure relates to the field of computer system manufacturing and computer system operations. More specifically, this disclosure relates to reducing an effect of signal distortion from reflection on transmission lines.
Computer systems have attained widespread use for providing computing power to many segments of today's modem society. A personal computer system can usually be defined as a desk top, floor standing, or portable microcomputer that includes a system unit having a system processor and associated volatile and non-volatile memory, a display monitor, a keyboard, one or more diskette drives, a fixed disk storage device and an optional printer. One of the distinguishing characteristics of these systems is the use of a system board to connect these components together electrically. These personal computer systems are information handling systems which are designed primarily to give independent computing power to a single user (or a group of users in the case of personal computers which serve as computer server systems) and are inexpensively priced for purchase by individuals or small businesses.
A personal computer system may also include one or more of a plurality of input/output (“I/O”) devices that are coupled to the system processor and that perform specialized functions. Examples of I/O peripheral devices include modems, sound and video devices, or specialized communication devices. Mass storage devices such as hard disks, compact disk (“CD”) drives of many varieties, magneto-optical drives, and other data storage devices are also considered to be peripheral devices.
In particular, many personal computer systems have monitors that provide the user of the system with a visual display. This visual display can provide, e.g., visual output or a visual interface for the operation of computer software.
Most personal computer systems based on the INTERNATIONAL BUSINESS MACHINES (“IBM®”) architecture use an interface cable to connect the monitor to the remainder of the personal computer system. This cable generally has a characteristic impedance of approximately 30 to 120 Ω and is terminated with an impedance of value Zt that can range from approximately 2.2 kΩ to 4.7 kΩ or more. This termination impedance is so far removed from the characteristic impedance of the cable that the cable is effectively unterminated. This condition gives rise to reflections of fast signals on the line, which in turn give rise to distortions of those signals. In particular, horizontal synchronization (“h-sync”) are sometimes distorted in this way.
In some computer systems involving specific items of equipment, these signal distortions on the h-sync line in the interface cable erratically trigger the h-sync processor. This condition manifests as “jitter,” wherein the visual display on the monitor screen is unstable in the horizontal axis. This jitter can be very distracting for a user, and in extreme cases the jitter makes it difficult or impossible for a user to read the image on the monitor screen.
One of the few practical methods of reducing this problem has been to provide an adapter plug for a particular computer system in which this problem has occurred, since the problem occurs in a relatively small number of computer system equipment combinations. The adapter plug contains a resistor in series with the h-sync signal to move the distorted signal to another voltage to separate it from the signal threshold voltage level of the h-sync signal receiver. However, manufacture of the adapter, shipping to customers when the problem occurs, and installation of the adapter are relatively expensive solutions. Because the problem occurs only rarely, this solution, to remain cost-effective, often requires allowing the problem to occur in the field (e.g., after sale to and installation for a customer) when particular monitors are used in particular computer system configurations, and only then applying the adapter. If the adapter is installed in every computer system shipped, the problem will simply occur in set of computer system configurations other than the set in which it would have occurred absent the adapters.
Another method of reducing this problem involves adding a filter circuit to such an adapter plug to lengthen the rise time of the h-sync signal sufficiently to eliminate the reflections. This solution, however, induces a new form of undesirable jitter, in addition to the drawbacks already mentioned.
What is needed is a system and method that reduces reducing effects of transmission line reflections, such as jitter in a computer system monitor display caused by reflections on the h-sync line to the monitor, that does not involve the expensive manufacture, shipping, and installation of adapters and that does not cause another type of undesirable jitter while reducing the original problem.